.start{
  display: flex;
  height: 100vh;
  //（弹性布局）垂直居中
  justify-content: center;
  //（弹性布局）水平居中
  align-items: center;
  //文字居中
  text-align: center;
  //子元素垂直排列
  flex-flow: column;
}


